High-strength galvanized steel sheet and method for producing the same

ABSTRACT

A high-strength galvanized steel sheet that includes a chemical composition containing, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and Fe and inevitable impurities. The steel sheet having a microstructure containing, by an area percentage basis, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases containing a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%).

TECHNICAL FIELD

The present disclosure relates to a high-strength galvanized steel sheetand a method for producing the high-strength galvanized steel sheet. Thehigh-strength galvanized steel sheet of the present disclosure issuitably used as a steel sheet for automobiles.

BACKGROUND ART

To reduce the amount of CO₂ emission in view of global environmentalconservation, an improvement in the fuel consumption of automobiles byreducing the weight of automobile bodies while maintaining the strengthof automobile bodies is always an important issue in the automobileindustry. In order to reduce the weight of automobile bodies whilemaintaining the strength thereof, it is effective to reduce thethickness of galvanized steel sheets used as materials for automobileparts by increasing the strength of steel sheets. Meanwhile, many ofautomobile parts composed of steel sheets are formed by press working orburring working, or the like. Therefore, it is desired thathigh-strength galvanized steel sheets used as materials for automobileparts have good formability in addition to desired strength.

In recent years, high-strength galvanized steel sheets having a tensilestrength (TS) of 1180 MPa or more have been increasingly used asmaterials for automobile body frames. High-strength galvanized steelsheets are required to have good bendability, good ductility, and inparticular, good uniform elongation for the formation thereof. Partscomposed of high-strength galvanized steel sheets are required to havehigh yield strength in view of crashworthiness, and it is extremelyimportant to achieve all these properties. Various high-strengthgalvanized steel sheets have been developed under these circumstances.

Patent Literature 1 discloses a high-strength galvanized steel sheethaving good bendability owing to the control of precipitates and atechnology, as a method for producing thereof, for controlling thecooling rate of molten steel prior to solidification, an annealingtemperature during annealing, and a subsequent cooling rate.

Patent Literature 2 discloses a high-strength galvanized steel sheethaving good ductility and good bendability owing to the control of thebalance between Si and Al, retained γ, and the Vickers hardness of aportion directly below a surface, and a technology, as a method forproducing thereof, for controlling a finishing temperature, a coilingtemperature, an annealing temperature range, a cooling rate afterannealing, a cooling stop temperature, and a cooling stop holding time.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2007-16319

PTL 2: Japanese Unexamined Patent Application Publication No.2009-270126

SUMMARY Technical Problem

In the technology described in Patent Literature 1, a precipitationhardened steel has high yield strength (YS) and low uniform elongation(UEL). There is no information about the steel in the range of a tensilestrength (TS) of 1180 MPa or more. There is no information about animprovement in the properties of the material by performing annealingmultiple times.

In the technology described in Patent Literature 2, the steel has a lowtensile strength (TS) less than 900 MPa, and there is room forimprovement therein. Furthermore, there is no information about animprovement in the properties of the material by performing annealingmultiple times.

The present disclosure has been accomplished in light of the foregoingproblems in the related art. It is an object of the present disclosureto provide a high-strength galvanized steel sheet having a tensilestrength (TS) of 1180 MPa or more, high yield strength (YS), gooduniform elongation, and good bendability, and a method for producing thehigh-strength galvanized steel sheet.

Solution to Problem

To overcome the foregoing problems and produce a galvanized steel sheethaving high strength, good uniform elongation, and good bendability, theinventors have conducted intensive studies from the viewpoint of thechemical composition and the microstructure of the steel sheet and amethod for producing the steel sheet, and have found the following.

A tensile strength (TS) of 1180 MPa or more, a yield strength (YS) of850 MPa or more, a uniform elongation of 7.0% or more, and goodbendability can be achieved by allowing a galvanized steel sheet tocontain, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% ormore and 2.5% or less, Mn: 2.3% or more and 4.0% or less, and Al: 0.01%or more and 2.5% or less; and performing hot rolling, cold rolling, andannealing under appropriate conditions to allow the galvanized steelsheet to contain, on an area percentage basis, a tempered martensitephase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68%or less, a retained austenite phase: 2% or more and 20% or less, andother phases: 10% or less (including 0%), the other phases containing amartensite phase: 3% or less (including 0) and a bainitic ferrite phase:less than 5% (including 0%), the tempered martensite phase having anaverage grain size of 8 μm or less, and the retained austenite phasehaving a C content less than 07% by mass.

The present disclosure has been made on the basis of these findings.Exemplary embodiments of the present disclosure are described below.

[1] A high-strength galvanized steel sheet includes a chemicalcomposition containing, by mass %, C: 0.15% or more and 0.25% or less,Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less,P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less,and the balance being Fe and inevitable impurities; and

a steel-sheet microstructure containing, in terms of area fraction, atempered martensite phase: 30% or more and 73% or less, a ferrite phase:25% or more and 68% or less, a retained austenite phase: 2% or more and20% or less, and other phases: 10% or less (including 0%), the otherphases containing a martensite phase: 3% or less (including 0%) and abainitic ferrite phase: less than 5% (including 0%), the temperedmartensite phase having an average grain size of 8 μm or less, and theretained austenite phase having a C content less than 0.7% by mass.

[2] In the high-strength galvanized steel sheet described in [1], thechemical composition further contains, by mass %, at least one elementselected from Cr: 0.01% or more and 2.0% or less, Ni: 0.01% or more and2.0% or less, and Cu: 0.01% or more and 2.0% or less.

[3] In the high-strength galvanized steel sheet described in [1] or [2],the chemical composition further contains, by mass %, B: 0.0002% or moreand 0.0050% or less.

[4] In the high-strength galvanized steel sheet described in any one of[1] to [3], the chemical composition further contains, by mass %, atleast one element selected from Ca: 0.001% or more and 0.005% or lessand REM: 0.001% or more and 0.005% or less.

[5] In the high-strength galvanized steel sheet described in any one of[1] to [4], the galvanized steel sheet includes a galvannealed steelsheet.

[6] In the high-strength galvanized steel sheet described in any one of[1] to [5], the galvanized steel sheet has a tensile strength of 1180MPa or more.

[7] A method for producing a high-strength galvanized steel sheetincludes:

a hot-rolling step of allowing a slab having the chemical compositiondescribed in any of [1] to [4] to have a temperature of 1100° C. orhigher, hot-rolling the slab at a finish rolling temperature of 800° C.or higher to produce a hot-rolled steel sheet, and coiling thehot-rolled steel sheet at a coiling temperature of 550° C. or lower;

a cold-rolling step of cold-rolling the hot-rolled steel sheet at acumulative rolling reduction of more than 20% to produce a cold-rolledsteel sheet;

a pre-annealing step of heating the cold-rolled steel sheet to anannealing temperature of 800° C. or higher and 1000° C. or lower,holding the cold-rolled steel sheet at the annealing temperature for 10s or more, and cooling the cold-rolled steel sheet to a cooling stoptemperature of 550° C. or lower at an average cooling rate of 5° C./s ormore;

a final-annealing step of heating the cold-rolled steel sheet to 680° C.or higher and 790° C. or lower at an average heating rate of 10° C./s orless, holding the cold-rolled steel sheet at the annealing temperaturefor 30 s or more and 1000 s or less, and cooling the cold-rolled steelsheet to a cooling stop temperature of 460° C. or higher and 550° C. orlower at an average cooling rate of 1.0° C./s or more, and holding thecold-rolled steel sheet at the cooling stop temperature for 500 s orless;

a galvanization step of galvanizing the cold-rolled steel sheet that hasbeen subjected to final annealing and cooling the galvanized cold-rolledsteel sheet to room temperature; and

a tempering step of tempering the galvanized cold-rolled steel sheet ata tempering temperature of 50° C. or higher and 400° C. or lower.

[8] In the method for producing a high-strength galvanized steel sheetdescribed in [7], the galvanization step includes, after thegalvanization, galvannealing treatment in which the galvanizedcold-rolled steel sheet is held in a temperature range of 460° C. orhigher and 580° C. or lower for 1 s or more and 120 s or less and thencooled to room temperature.

[9] The method for producing a high-strength galvanized steel sheetdescribed in [7] or [8] further includes before and/or after thetempering step, a temper-rolling step of performing tempering at anelongation percentage of 0.05% or more and 1.00% or less.

It should be noted that in the present disclosure, the “high-strengthgalvanized steel sheet” refers to a steel sheet having a tensilestrength of 1180 MPa or more and includes a galvannealed steel sheet inaddition to a galvanized steel sheet. Furthermore, galvanizing includesgalvannealing in addition to galvanizing. In the case where a galvanizedsteel sheet and a galvannealed steel sheet need to be distinctivelyexplained, these steel sheets are distinctively described.

Advantageous Effects

According to the present disclosure, it is possible to obtain ahigh-strength galvanized steel sheet having a tensile strength (TS) of1180 MPa or more, a high yield strength (YS) of 850 MPa or more, auniform elongation of 7.0% or more, and good bendability in which whenthe steel sheet is bent at a bending radius of 2.0 mm, the length of acrack is 0.5 mm or less. The high-strength galvanized steel sheet issuitable as a material for automobile parts.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present disclosure are described in detailbelow. The symbol “%” that expresses the content of a component elementrefers to “% by mass” unless otherwise specified.

1) Chemical Composition C: 0.15% or More and 0.25% or Less

C is an element needed to form a martensite phase or increase thehardness of a martensite phase to increase tensile strength (TS),Furthermore, C is an element needed to stabilize a retained austenitephase to obtain uniform elongation. When the C content is less than0.15%, the martensite phase has low strength, and the retained austenitephase is insufficiently stabilized. It is thus difficult to achieve botha tensile strength (TS) of 1180 MPa or more and a uniform elongation of7.0% or more. When the C content is more than 0.25%, bendability ismarkedly degraded. Accordingly, the C content is 0.15% or more and 0.25%or less. The lower limit is preferably 0.16% or more. The upper limit ispreferably 0.22% or less.

Si: 0.50% or More and 2.5% or Less

Si is an element effective in increasing the tensile strength (TS) ofsteel by solid solution strengthening. Furthermore, Si is an elementeffective in inhibiting the formation of cementite to provide a retainedaustenite phase. To achieve these effects, it is necessary to set the Sicontent to be 0.50% or more. An increase in Si content leads to theexcessive formation of a ferrite phase to cause the degradation of thebendability, the coatability, and the weldability. Thus, an appropriateaddition is preferred. Accordingly, the Si content is 0.50% or more and2.5% or less, preferably 0.50% or more and 2.0% or less, and morepreferably 0.50% or more and 1.8% or less.

Mn: 2.3% or More and 4.0% or Less

Mn is an element that increases the tensile strength (TS) of steel bysolid solution strengthening, that inhibits ferrite transformation andbainite transformation, and that forms a martensite phase to increasethe tensile strength (TS). Furthermore, Mn is an element that stabilizesan austenite phase to increase uniform elongation. To sufficientlyprovide these effects, the Mn content needs to be 2.3% or more. When theMn content is more than 4.0%, inclusions increase significantly to causethe degradation of the bendability. Accordingly, the Mn content is 2.3%or more and 4.0% or less. The Mn content is preferably 2.3% or more and3.8% or less and more preferably 2.3% or more and 3.5% or less.

P: 0.100% or Less

P is desirably minimized as much as possible because the bendability andthe weldability are degraded by grain boundary segregation. Theallowable upper limit of the P content in the present disclosure is0.100%. The P content is preferably 0.050% or less and more preferably0.020% or less. The lower limit is not particularly specified andpreferably 0.001% or more because the P content of less than 0.001%leads to a reduction in production efficiency.

S: 0.02% or Less

S is present in the form of inclusions such as MnS and degradesweldability. Thus, the S content is preferably minimized as much aspossible. The allowable upper limit of the S content in the presentdisclosure is 0.02%. The S content is preferably 0.0040% or less. Thelower limit is not particularly specified and is preferably 0.0005% ormore because the S content of less than 0.0005% leads to a reduction inproduction efficiency.

Al: 0.01% or More and 2.5% or Less

Al is an element effective in stabilizing an austenite phase to obtain aretained austenite phase. When the Al content is less than 0.01%, it isnot possible to stabilize the austenite phase to obtain the retainedaustenite phase. When the Al content of more than 2.5%, a risk of slabcracking during the continuous casting increases and a ferrite phase isexcessively formed during annealing. It is thus difficult to achieveboth a tensile strength (TS) of 1180 MPa or more and good bendability.Accordingly, the Al content is 0.01% or more and 2.5% or less. The Alcontent is preferably 0.01% or more and 1.0% or less and more preferably0.01% or more and 0.6% or less from the viewpoint of inhibiting theexcessive formation of the ferrite phase.

The balance is Fe and inevitable impurities. One or more elementsdescribed below may be appropriately contained therein, as needed.

At Least One Element Selected from Cr: 0.01% or More and 2.0% or Less,Ni: 0.01% or More and 2.0% or Less, and Cu: 0.01% or More and 2.0% orLess

Cr, Ni, and Cu are elements that form low-temperature transformationphases such as a martensite phase and thus are effective in increasingstrength. To provide the effect, the content of at least one elementselected from Cr, Ni, and Cu is 0.01% or more. When the content of eachof Cr, Ni, and Cu is more than 2.0%, the effect is saturated.Accordingly, when these elements are contained, the content of each ofCr, Ni, and Cu is 0.01% or more and 2.0% or less. The lower limit of thecontent of each of the elements is preferably 0.1% or more. The upperlimit is preferably 1.0% or less.

B: 0.0002% or More and 0.0050% or Less

B is an element that segregates to grain boundaries and thus iseffective in inhibiting the formation of a ferrite phase and a bainitephase to promote the formation of a martensite phase. To sufficientlyprovide the effects, the B content needs to be 0.0002% or more. When theB content is more than 0.0050%, the effects are saturated to lead tocost increases. Accordingly, when B is contained, the B content is0.0002% or more and 0.0050% or less. The lower limit of the B content ispreferably 0.0005% or more in view of the formation of the martensitephase. The upper limit is preferably 0.0030% or less and more preferably0.0020% or less.

At Least One Element Selected from Ca: 0.001% or More and 0.005% or Lessand REM: 0.001% or More and 0.005% or Less

Ca and REM are elements effective in controlling the form of sulfides toimprove formability. To provide the effect, the content of at least oneelement selected from Ca and REM is 0.001% or more. When each of the Cacontent and the REM content is more than 0.005%, the cleanliness ofsteel might be adversely affected to degrade the properties.Accordingly, when these elements are contained, the content of each ofCa and REM is 0.001% or more and 0.005% or less.

2) Microstructure of Steel Sheet Area Fraction of Tempered MartensitePhase: 30% or More and 73% or Less

When the area fraction of a tempered martensite phase is less than 30%,it is difficult to achieve both a high tensile strength (TS) of 1180 MPaor more and high bendability. When the area fraction of a temperedmartensite phase is more than 73%, uniform elongation of the presentdisclosure is not provided. Accordingly, the area fraction of thetempered martensite phase is 30% or more and 73% or less. The lowerlimit of the area fraction is preferably 40% or more. The upper limit ispreferably 70% or less and more preferably 65% or less.

Area Fraction of Ferrite Phase: 25% or More and 68% or Less

When the area fraction of a ferrite phase is less than 25%, the uniformelongation of the present disclosure is not obtained. When the areafraction of the ferrite phase is more than 68%, the yield strength (YS)of the present disclosure is not obtained. Accordingly, the areafraction of the ferrite phase is 25% or more and 68% or less. The lowerlimit of the area fraction is preferably 35% or more. The upper limit ispreferably 60% or less. An unrecrystallized ferrite phase, which isundesirable for the ductility and the bendability, is not included inthe ferrite phase in the present disclosure.

Area Fraction of Retained Austenite Phase: 2% or More and 20% or Less

When the area fraction of a retained austenite phase is less than 2%, itis difficult to achieve both a high tensile strength (TS) of 1180 MPa ormore and a uniform elongation of 7.0% or more. When the area fraction ofthe retained austenite phase is more than 20%, the bendability isdegraded. Accordingly, the area fraction of the retained austenite phaseis 2% or more and 20% or less. The lower limit of the area fraction ispreferably 3% or more. The upper limit is preferably 15% or less.

The microstructure of the steel sheet of the present disclosure may becomposed of three phases: a tempered martensite phase, a ferrite phase,and a retained austenite phase. Other phases may be allowable as long asthe area fraction of the other phases is 10% or less. Because the otherphases are undesirable for the bendability and the tensile strength(TS), the area fraction of the other phases is 10% or less, preferablyless than 5%, and more preferably less than 3%. Examples of the otherphases include a martensite phase, a bainitic ferrite phase, a pearlitephase, and an unrecrystallized ferrite phase.

Regarding the other phases, the allowable area fraction of each of themartensite phase and the bainitic ferrite phase is specified from thefollowing reasons.

Area Fraction of Martensite Phase: 3% or Less (Including 0%)

When the area fraction of the martensite phase is more than 3%, thebendability are markedly degraded. Accordingly, the area fraction of themartensite is 3% or less, preferably 2% or less, and more preferably 1%or less.

Area Fraction of Bainitic Ferrite Phase: Less than 5% (Including 0%)

When the area fraction of the bainitic ferrite phase is 5% or more, itis difficult to achieve both a high tensile strength (TS) of 1180 MPa ormore and a uniform elongation of 7.0% or more. Accordingly, the areafraction of the bainitic ferrite phase is less than 5%.

Average Grain Size of Tempered Martensite Phase: 8 μm or Less

When the average grain size of the tempered martensite phase is morethan 8 μm, the bendability is markedly degraded. Accordingly, theaverage grain size of the tempered martensite is 8 μm or less. Theaverage grain size is preferably 4 μm or less. Note that the grains ofthe tempered martensite phase in the present disclosure refer to grainsof a tempered martensite phase surrounded by grain boundaries of a prioraustenite phase or grain boundaries of a tempered martensite phase andother phases, such as a ferrite phase and a bainitic ferrite phase.

C Content of Retained Austenite Phase: Less than 0.7% by Mass

When the C content of the retained austenite phase is 0.7% by mass ormore, the retained austenite phase is excessively stable. It is thusdifficult to achieve both a high tensile strength (TS) of 1180 MPa ormore and a uniform elongation of 7.0% or more. Accordingly, the Ccontent of the retained austenite phase is less than 0.7% by mass andpreferably 0.6% by mass. The C content of the retained austenite phasein the present disclosure is a value determined by X-ray diffraction.

The area fraction of each of the martensite phase, the temperedmartensite phase, the ferrite phase, the bainitic ferrite phase, thepearlite phase, and the unrecrystallized ferrite phase in the presentdisclosure refers to the fraction of the area of the corresponding phasewith respect to an observed area. The area fraction of each phase isdetermined by cutting samples from a steel sheet that has been subjectedto a final production step, polishing a section parallel to a rollingdirection, etching the section with a 3% nital, capturing images ofthree fields of view, for each sample, of a portion of the steel sheetbelow a zinc coat using a scanning electron microscope (SEM) at amagnification of ×1500, the portion being located away from a surface ofthe steel sheet by ¼ of the sheet thickness of the steel sheet,determining the area fraction of each phase from the resulting imagedata using Image-Pro from Media Cybernetics, Inc., and defining theaverage area fraction of each phase in the fields of view as the areafraction of each phase. The phases can be distinguished from each otherin the image data as follows: the ferrite phase appears black, themartensite phase appears white and does not include a carbide, atempered martensite phase appears gray and includes a carbide, thebainitic ferrite phase appears dark gray and includes a carbide or amartensite island phase, and the pearlite appears as black-and-whitelayers. The unrecrystallized ferrite phase appears black, includessub-boundaries, and thus is distinguished from the ferrite phase.Because the martensite island phase is regarded as a tempered martensitephase in the present disclosure, the area fraction of the temperedmartensite phase includes the area fraction of the martensite islandphase.

The average grain size of the tempered martensite phase is determined asfollows: The total area of the tempered martensite phase in the fieldsof view in the image data used for the determination of the areafraction is divided by the number of the tempered martensite phase todetermine the average area. The square root of the average area isdefined as the average grain size thereof.

The area fraction of the retained austenite phase is determined asfollows: A steel sheet that has been subjected to a final productionstep is ground in such a manner that a portion of the steel sheet belowthe zinc coat, the portion being located away from a surface of thesteel sheet by ¼ of the sheet thickness of the steel sheet, is ameasurement surface. The steel sheet is further polished to a depth of0.1 mm by chemical polishing. In the measurement surface located awayfrom the surface of the steel sheet by ¼ of the sheet thickness, theintegrated intensities of reflections from the (200), (220), and (311)planes of fcc iron (austenite phase) and the (200), (211), and (220)planes of bcc iron (ferrite phase) are measured with an X-raydiffractometer using MoKα radiation. The volume fraction is determinedfrom the intensity ratio of the integrated intensity of reflection fromthe planes of fcc iron (austenite phase) to the integrated intensity ofreflection from the planes of bcc iron (ferrite phase). The value of thevolume fraction is used as a value of the area fraction. The C contentof the retained austenite phase is calculated from the amount of shiftof a diffraction peak corresponding to the (220) plane measured withX-ray diffractometer using CoKα radiation by means of the followingexpression:

a=1.7889×(2)^(1/2)/sin θ

a=3.578+0.033[C]+0.00095[Mn]+0.0006[Cr]+0.022[N]+0.0056[Al]+0.0015[Cu]+0.0031[Mo]

where a represents the lattice constant (Å) of the austenite, θrepresents a value obtained by dividing the diffraction peak angle (rad)corresponding to the (220) plane by 2, and [M] represents the content (%by mass) of an element M in the austenite (an element that is notcontained is zero). The content (% by mass) of the element M in theretained austenite phase in the present disclosure is content withrespect to the entire steel.

3) Applications and so Forth of Steel Sheet

Applications of the high-strength galvanized steel sheet of the presentdisclosure are not particularly limited. The high-strength galvanizedsteel sheet is preferably used for automobile parts.

The thickness (excluding the coat) of the high-strength galvanized steelsheet of the present disclosure is not particularly limited and ispreferably 0.4 mm or more and 3.0 mm or less.

4) Production Conditions

The high-strength galvanized steel sheet of the present disclosure maybe produced by, for example, a production method including a hot-rollingstep of heating a slab having the chemical composition described aboveto a temperature of 1100° C. or higher, hot-rolling the slab at a finishrolling temperature of 800° C. or higher to produce a hot-rolled steelsheet, and coiling the hot-rolled steel sheet at a coiling temperatureof 550° C. or lower; a cold-rolling step of cold-rolling the hot-rolledsteel sheet at a cumulative rolling reduction of more than 20% toproduce a cold-rolled steel sheet; a pre-annealing step of heating thecold-rolled steel sheet to an annealing temperature of 800° C. or higherand 1000° C. or lower, holding the cold-rolled steel sheet at theannealing temperature for 10 s or more, and cooling the cold-rolledsteel sheet to a cooling stop temperature of 550° C. or lower at acooling stop temperature of 5° C./s or more; a final-annealing step ofheating the cold-rolled steel sheet to an annealing temperature of 680°C. or higher and 790° C. or lower at an average heating rate of 10° C./sor less, holding the cold-rolled steel sheet at the annealingtemperature for 30 s or more and 1000 s or less, and cooling thecold-rolled steel sheet to a cooling stop temperature of 460° C. orhigher and 550° C. or lower at an average cooling rate of 1.0° C./s ormore, and holding the cold-rolled steel sheet at the cooling stoptemperature for 500 s or less; a galvanization step of galvanizing thecold-rolled steel sheet subjected to final annealing and, optionally,further heating the galvanized cold-rolled steel sheet to 460° C. to580° C. to perform galvannealing treatment, and cooling the galvanizedcold-rolled steel sheet to room temperature; and a tempering step oftempering the galvanized cold-rolled steel sheet at a temperingtemperature of 50° C. or higher and 400° C. or lower. The details willbe described below.

4-1) Hot-Rolling Step Slab Temperature: 1100° C. or Higher

A slab temperature of lower than 1100° C. results in an unmeltedcarbide, thereby failing to obtain the microstructure of the steel sheetof the present disclosure. Accordingly, the slab heating temperature is1100° C. or higher. To suppress an increase in scale loss, the heatingtemperature of the slab is preferably 1300° C. or lower. In thehot-rolling step of the present disclosure, the temperature of amaterial, such as a slab, to be rolled refers to the surface temperatureof a portion of the material, such as a slab, to be rolled, the portionbeing located in the middle of the material in the longitudinal andtransverse directions.

The slab is preferably produced by a continuous casting process toprevent macro-segregation and may also be produced by an ingot-makingprocess or a thin slab casting process. To hot-rolling the slab, theslab may be temporarily cooled to room temperature, reheated, andsubjected to hot rolling. The slab may be placed in a heating furnacewithout cooling to room temperature to perform hot rolling. Anenergy-saving process may be employed in which hot rolling is performedimmediately after the slab is slightly heated.

Finish Rolling Temperature: 800° C. or Higher

When the finish rolling temperature is lower than 800° C., because aferrite phase and so forth is formed, rolling is performed in atwo-phase region to form a nonuniform microstructure of the steel sheet,thereby failing to obtain the microstructure of the steel sheet of thepresent disclosure. Accordingly, the finish rolling temperature is 800°C. or higher. The upper limit of the finish rolling temperature is notparticularly limited and is preferably 950° C. or lower.

When the slab is hot-rolled, a rough bar after rough rolling may beheated in order to prevent the occurrence of a trouble even if theheating temperature of the slab is reduced. A continuous rolling processmay also be employed in which rough bars are joined together and finishrolling is continuously performed. The finish rolling can increase theanisotropy to reduce the workability after the cold rolling andannealing and thus is preferably performed at a finishing temperatureequal to or higher than the Ar₃ transformation point. To reduce therolling load and uniformize the shape and the material properties, allor some passes of the finish rolling is preferably replaced withlubrication rolling at a coefficient of friction of 0.10 to 0.25.

Coiling Temperature: 550° C. or Lower

A coiling temperature higher than 550° C. results in the formation ofsoft phases, such as a ferrite phase and a pearlite phase, in ahot-rolled coil, thus failing to obtain the microstructure of the steelsheet of the present disclosure. Accordingly, the coiling temperature is550° C. or lower. The lower limit is not particularly specified and ispreferably 400° C. or higher because a coiling temperature lower than400° C. results in the degradation of the form of the sheet.

The hot-rolled steel sheet that has been coiled is subjected to picklingto remove scales. Then the hot-rolled steel sheet is subjected to coldrolling, pre-annealing, final annealing, galvanization, tempering, andso forth under conditions described below.

4-2) Cold-Rolling Step

Cumulative Rolling Reduction: More than 20%

A cumulative rolling reduction of 20% or less is liable to lead to theformation of coarse grains during annealing, thereby failing to obtainthe microstructure of the steel sheet of the present disclosure.Accordingly, the cumulative rolling reduction in the cold rolling ismore than 20% and preferably 30% or more. The upper limit is notparticularly specified and is preferably about 90% or less and morepreferably 75% or less in view of the stability of the form of thesheet.

In the present disclosure, the microstructure is formed by thepre-annealing step, the final annealing step, and the tempering step. Inthe pre-annealing step, after the formation of an austenite phase,cooling is performed so as to inhibit soft microstructures, such as aferrite phase and a pearlite phase, thereby forming hard microstructuresincluding a bainite phase, a martensite phase, and so forth.Subsequently, in the final annealing step, a fine austenite phase isformed on a hard microstructure basis by controlling the heating rateand the annealing temperature. Furthermore, the bainite phase isinhibited by controlling the holding of the cooling, thereby forming aretained austenite phase and the martensite phase that have low Ccontents. In the tempering step, the martensite phase is tempered toform a tempered martensite phase. The details will be described below.

4-3) Pre-Annealing Step Annealing Temperature: 800° C. or Higher and1000° C. or Lower

When the annealing temperature in the pre-annealing step s lower than800° C., the austenite phase is insufficiently formed, thereby failingto obtain the microstructure of the steel sheet of the presentdisclosure. When the annealing temperature in the pre-annealing step ishigher than 1000° C., coarse austenite grains are formed, therebyfailing to obtain the microstructure of the steel sheet of the presentdisclosure. Accordingly, the annealing temperature in the pre-annealingstep is 800° C. or higher and 1000° C. or lower and preferably 800° C.or higher and 930° C. or lower.

Holding Time of 10 s or More at Annealing Temperature

When the annealing holding time in the pre-annealing is less than 10 s,austenite is insufficiently formed, thereby failing to obtain themicrostructure of the steel sheet of the present disclosure.Accordingly, the annealing holding time in the ore-annealing step is 10s or more. The annealing holding time in the pre-annealing is notparticularly limited and is preferably 500 s or less.

Average Cooling Rate: 5° C./s or More

When the average cooling rate is less than 5° C./s from the annealingtemperature to 550° C., a ferrite phase and a pearlite phase are formedduring cooling, thereby failing to obtain the microstructure of thesteel sheet of the present disclosure. Accordingly, the average coolingrate is 5° C./s or more and preferably 8° C./s or more at temperaturesup to 550° C.

In the present disclosure, the symbol “s” used as the unit of each ofthe average heating rate and the average cooling rate refers to“seconds”.

Cooling Stop Temperature: 550° C. or Lower

When the cooling stop temperature at an average cooling rate of 5° C./sor more is higher than 550° C., large amounts of the ferrite phase andthe pearlite phase are formed, thereby failing to obtain themicrostructure of the steel sheet of the present disclosure.Accordingly, the cooling stop temperature at an average cooling rate of5° C./s or more is 550° C. or lower. In the case of a cooling stoptemperature lower than 550° C., the cooling rate at temperatures lowerthan 550° C. is not particularly limited and may be less than 5° C./s.The lower limit of the cooling stop temperature may be appropriatelyadjusted and is preferably 0° C. or higher. In the present disclosure,reheating may be performed after cooling in the pre-annealing step. Thereheating temperature is preferably 50° C. to 550° C. from the viewpointof inhibiting the ferrite phase and the pearlite phase. The holding timeat the reheating temperature is preferably 1 to 1000 s from theviewpoint of inhibiting the ferrite phase and the pearlite phase. Afterthe pre-annealing, cooling is performed to 50° C. or lower.

4-4) Final Annealing Step Average Heating Rate: 10° C./s or Less

When the average heating rate is more than 10° C./s at temperatures upto the annealing temperature, coarse austenite is formed, therebyfailing to obtain the microstructure of the steel sheet of the presentdisclosure. Accordingly, the average heating rate is 10° C./s or less.The average heating rate is a value obtained by dividing the temperaturedifference between the annealing temperature and 200° C. by the heatingtime it takes for the temperature to increase from 200° C. to theannealing temperature.

Annealing Temperature: 680° C. or Higher and 790° C. or Lower

An annealing temperature lower than 680° C. results in insufficientformation of an austenite phase thereby failing to the microstructure ofthe steel sheet of the present disclosure. An annealing temperature ofhigher than 790° C. results in excessive formation of the austenitephase, thereby failing to the microstructure of the steel sheet of thepresent disclosure. Accordingly, the annealing temperature is 680° C. orhigher and 790° C. or lower and preferably 700° C. or higher and 790° C.or lower.

Holding Time of 30 s or More and 1000 s or Less at Annealing Temperature

An annealing holding time of less than 30 s results in insufficientformation of an austenite phase, thereby failing to the microstructureof the steel sheet of the present disclosure. An annealing holding timeof more than 1000 s results in the formation of a coarse austenitephase, thereby failing to the microstructure of the steel sheet of thepresent disclosure. Accordingly, the annealing holding time is 30 to1000 s. The lower limit of the annealing holding time is preferably 60 sor more. The upper limit is preferably 600 s or less.

Average Cooling Rate: 1.0° C./s or More

When the average cooling rate is less than 1.0° C./s at temperaturesfrom the annealing temperature to the cooling stop temperature, largeamounts of an ferrite phase and a pearlite phase or a bainite phase areformed during cooling, thereby failing to the microstructure of thesteel sheet of the present disclosure. Accordingly, the average coolingrate is 1.0° C./s or more. The upper limit of the average cooling rateis not particularly limited, may be appropriately adjusted, andpreferably 100° C./s or less.

Cooling Stop Temperature: 460° C. or Higher and 550° C. or Lower

When the cooling stop temperature at an average cooling rate of 1.0°C./s or more is lower than 460° C., a large amount of a bainite phase isformed, thereby failing to the microstructure of the steel sheet of thepresent disclosure. When the cooling stop temperature is higher than550° C., large amounts of a ferrite phase and a pearlite phase areformed, thereby failing to the microstructure of the steel sheet of thepresent disclosure. Accordingly, the cooling stop temperature is 460° C.or higher and 550° C. or lower.

Cooling Stop Holding Time: 500 s or Less

A cooling stop holding time of more than 500 s results in the formationof large amounts of a bainite phase and a pearlite phase, therebyfailing to the microstructure of the steel sheet of the presentdisclosure. Accordingly, the cooling stop holding time is 500 s or less.The lower limit of the cooling stop holding time is not particularlylimited and may be appropriately adjusted. The cooling stop holding timemay be zero. The term “cooling stop holding time” used here refers tothe time the time from cooling to 550° C. or lower after annealing toimmersion in a galvanizing bath. The temperature at the time of thestoppage of the cooling need not be exactly maintained after thestoppage of the cooling. Cooling and heating may be performed as long asthe temperature range is 460° C. or higher and 550° C. or less.

4-5) Galvanization Step

Galvanization treatment is preferably performed by dipping the steelsheet that has been produced as described above in a galvanizing bathwith a temperature of 440° C. or higher and 500° C. or lower and thenadjusting the coating weight by, for example, gas wiping. When the zinccoat is alloyed, the alloying is preferably performed by holding thegalvanized steel sheet at a temperature range of 460° C. or higher and580° C. or lower for 1 s or more and 120 s or less. In thegalvanization, the galvanizing bath having an Al content of 0.08% ormore by mass and 0.25% or less by mass is preferably used.

The steel sheet subjected to galvanization may be subjected to any ofvarious coating treatments, such as resin coating and oil/fat coating.

4-6) Tempering Step Tempering Temperature: 50° C. or Higher and 400° C.or Lower

A tempering temperature lower than 50° C. results in insufficienttempering of the martensite phase, thereby failing to the microstructureof the steel sheet of the present disclosure. A tempering temperature ofhigher than 400° C. results in the decomposition of the austenite phase,thereby failing to the microstructure of the steel sheet of the presentdisclosure. Accordingly, the tempering temperature is 50° C. or higherand 400° C. or lower. The lower limit of the tempering temperature ispreferably 100° C. or higher. The upper limit is preferably 350° C. orlower. The tempering treatment may be performed with any of a continuousannealing furnace, a box annealing furnace, and so forth. When the steelsheet is subjected to tempering treatment in the form of a coil, thatis, when the steel sheet is in contact with itself, the tempering timeis preferably 24 hours or less in view of, for example, the inhibitionof adhesion.

4-7) Temper Rolling Step Elongation Rate: 0.05% or More and 1.00% orLess

In the present disclosure, temper rolling may be performed at anelongation rate of 0.05% or more and 1.00% or less before and/or afterthe tempering step. This temper rolling increases the yield strength(YS). The effect is provided at an elongation rate of 0.05% or more. Anelongation rate of more than 1.00% might result in a reduction inuniform elongation. Accordingly, when the temper rolling step isperformed, the elongation rate in the temper rolling is 0.05% or moreand 1.00% or less.

EXAMPLES

Exemplary examples are described below. The technical scope of thepresent disclosure is not limited to the following examples. Table 2-1and Table 2-2 are collectively referred to as Table 2.

Molten steels having chemical compositions listed in Table 1 were formedin a vacuum melting furnace and rolled into steel slabs. Underconditions listed in Table 2, these steel slabs were subjected toheating, rough rolling, finish rolling, cooling, and a processequivalent to coiling to provide hot-rolled steel sheets. Each of thehot-rolled steel sheets was subjected to cold rolling to a thickness of1.4 mm to provide a cold-rolled steel sheet, followed by pre-annealingand final annealing. The cold-rolled steel sheet was subjected togalvanization (with a galvanizing bath containing an Al content of 0.08%or more by mass and 0.25% or less by mass), cooling to room temperature,and tempering treatment. In some cases, temper rolling was performedbefore and/or after tempering treatment. Steel sheets 1 to 37 wereproduced under those conditions. The annealing was performed in alaboratory with a simulated continuous galvanizing line under conditionslisted in Table 2 to produce galvanized steel sheets (GI) andgalvannealed steel sheets (GA). The galvanized steel sheets wereproduced by immersing the steel sheets in a galvanizing bath with atemperature of 460° C. to form a coated layer at a coating weight of 35to 45 g/m². The galvannealed steel sheets were produced by performinggalvanization and then galvannealing in the range of 460° C. to 580° C.The tensile properties, the bendability, and the impact resistance ofthe resulting galvanized and galvannealed steel sheets were determinedaccording to the following test methods. Table 3 lists the results.

<Tensile Test>

A JIS No. 5 test piece for a tensile test (JIS Z 2201) was taken fromeach of the steel sheets that had been subjected to a final productionstep, in a direction perpendicular to a rolling direction. A tensiletest according to JIS Z 2241 was performed at a strain rate of 10⁻³/s todetermine the tensile strength (TS), the yield strength (YS), and theuniform elongation (UEL). A steel sheet having a tensile strength (TS)of 11.80 MPa or more, a yield strength (YS) of 850 MPa or more, and auniform elongation (UEL) of 7.0% or more was rated pass.

<Bending Test>

A strip test piece having a width of 35 mm and a length of 100 mm wastaken from each of the annealed sheets in such a manner that a directionparallel to the rolling direction was the direction of a bending axisfor the test, and was subjected to a bending test. Specifically, a 90°V-bending test was performed at stroke speed of 10 mm/s, a pressing loadof 10 tons, a press-holding time of 5 s, and a bending radius R of 2.0mm. A ridge portion at the apex of the resulting bend was observed usinga magnifier with a magnification of ×10. At one point in the widthdirection, a steel sheet in which a crack having a length of 0.5 mm wasformed was rated poor, and a steel sheet in which a crack having alength less than 0.5 mm was formed was rated good.

TABLE 1 Chemical composition (% by mass) Steel C Si Mn P S Al N OthersRemarks A 0.15 130 2.9 0.012 0.0012 0.031 0.002 — within scope ofdisclosure B 0.17 0.90 3.1 0.009 0.0018 0.033 0.003 — within scope ofdisclosure C 0.23 1.50 2.5 0.015 0.0011 0.035 0.002 — within scope ofdisclosure D 0.20 1.60 2.6 0.011 0.0022 0.410 0.003 — within scope ofdisclosure E 0.18 1.20 3.0 0.005 0.0041 0.019 0.002 Cr: 0.40 withinscope of disclosure F 0.16 0.60 3.5 0.013 0.0020 0.022 0.001 B: 0.0023within scope of disclosure G 0.17 1.40 31 0.025 0.0009 0.031 0.002 Ni:0.20 within scope of disclosure H 0.20 1.80 2.6 0.003 0.0023 0.038 0.003Cu: 0.30 within scope of disclosure I 0.18 1 80 2.7 0.015 0.0048 0.0250.002 Ca: 0.001 within scope of disclosure J 0.19 1.90 2.8 0.007 0.00170.044 0.002 REM: 0.002 within scope of disclosure K 0.13 1.30 2.6 0.0090.0014 0.029 0.003 — outside scope of disclosure L 0.28 1.70 2.5 0.0150.0008 0.030 0.003 — outside scope of disclosure M 0.17 0.20 2.7 0.0190.0021 0.035 0.003 — outside scope of disclosure N 0.21 1.70 2.1 0.0110.0016 0.010 0.002 — outside scope of disclosure O 0.22 0.70 2.9 0.0160.0027 2.700 0.004 — outside scope of disclosure P 0.15 1.30 4.5 0.0120.0012 0.031 0.002 — outside scope of disclosure

TABLE 2-1 Cold-rolling Pre-annealing conditions Hot-rollinng conditionsconditions Anneal- Anneal- Cooling Cooling Re- Re- Slab Finish CoilingCumulative ing ing Average stop stop heating heating Steel heatingrolling temp- rolling temp- holding cooling temp- holding temp- holdingsheet temperature temperature erature reduction erature time rateerature time erature time No. Steel (° C.) (° C.) (° C.) (%) (° C.) (s)(° C./s) (° C.) (s) (° C.) (s)  1 A 1200 880 550 50 850 180 10 450 300 ——  2 1200 880 550 50 750 180 10 450 300 — —  3 1200 880 550 50 1050 18010 450 300 — —  4 1200 880 550 50 850 180 1 450 300 — —  5 1200 880 55050 850 180 10 600 300 — —  6 B 1200 870 500 56 810 300 8 350 900 450 60 7 1200 870 500 56 810 5 8 350 900 450 60  8 1200 870 500 56 810 300 8350 900 450 60  9 1200 870 500 56 810 300 8 350 900 450 60 10 1200 870500 56 810 300 8 350 900 450 60 11 1200 870 500 56 810 300 8 350 900 45060 12 C 1200 890 450 53 880 120 15 500 180 — — 13 1200 890 450 53 880120 6 500 180 — — 14 1200 890 450 53 880 120 15 500 180 — — 15 1200 890450 53 880 120 15 500 180 — — 16 D 1200 900 450 42 900 180 20 250 120 —— 17 1200 900 450 42 900 180 20 250 120 — — 18 1200 900 450 42 900 18020 250 120 — — 19 1200 900 600 42 900 180 20 250 120 — — 20 E 1200 900500 43 870 60 12 400 300 — — 21 1200 900 500 43 870 60 12 400 300 — — 221200 900 500 10 870 60 12 400 300 — — 23 1200 900 500 43 870 60 12 400300 — — 24 F 1200 880 550 61 880 30 10 200 600 — — 25 1200 880 550 61880 30 10 200 600 — — 26 1200 880 550 61 880 30 10 200 600 — — 27 G 1200880 500 65 850 480 1000 25 — — — 28 1200 880 500 65 850 480 1000 25 5400 600 29 H 1200 880 500 36 850 120 15 400 300 — — 30 I 1200 880 500 36850 120 15 250 10 500 300 31 J 1200 880 500 50 850 120 15 200 10 300 60032 K 1200 880 500 50 850 120 15 400 300 — — 33 L 1200 880 500 50 850 12015 400 300 — — 34 M 1200 880 500 50 850 120 15 400 300 — — 35 N 1200 880500 53 850 120 15 400 300 — — 36 O 1200 950 500 50 950 300 50 400 300 —— 37 P 1200 880 550 50 850 180 10 450 300 — —

TABLE 2-2 Final annealing conditions Temper Temper Cooling Coatingconditions rolling after rolling after Average stop Alloying coatingTempering conditions tempering Steel heating Annealing Annealing AverageCooling stop holding coating bath Alloying holding Elongation TemperingTempering Elongation sheet rate temperature holding time cooling ratetemperature time temperature temperature time Coated rate temperaturetime rate No. Steel (° C./s) (° C.) (s) (° C./s) (° C.) (s) (° C.) (°C.) (s) state* (%) (° C.) (s) (%) Remarks  1 A 5 740 120 10 500 60 465550 20 GA — 250 7200 — Example  2 5 740 120 10 500 60 465 550 20 GA —250 7200 — Comparative example  3 5 740 120 10 500 60 465 550 20 GA —250 7200 — Comparative example  4 5 740 120 10 500 60 465 550 20 GA —250 7200 — Comparative example  5 5 740 120 10 500 60 465 550 20 GA —250 7200 — Comparative example  6 B 3 760 300 8 460 30 465 — 10 GI — 2004800 — Example  7 3 760 300 8 460 30 465 — 10 GI — 200 4800 —Comparative example  8 30 760 300 8 460 30 465 — 10 GI _— 200 4800 —Comparative example  9 3 670 300 8 460 30 465 — 10 GI — 200 4800 —Comparative example 10 3 810 300 8 460 30 465 — 10 GI — 200 4800 —Comparative example 11 3 820 300 30 460 30 465 — 10 GI — 200 4800 —Comparative example 12 C 8 780 60 15 480 10 465 580 15 GA 0.20 200 72000— Example 13 8 780 5 15 480 10 465 580 15 GA 0.20 200 72000 —Comparative example 14 8 780 1500 15 480 10 465 580 15 GA 0.20 200 72000— Comparative example 15 8 780 60 0.5 480 10 465 580 15 GA 0.20 20072000 — Comparative example 16 D 5 770 120 30 500 30 465 520 20 GA — 25060 — Example 17 5 770 120 30 600 30 465 520 20 GA — 250 60 — Comparativeexample 18 5 770 120 30 500 900 465 520 20 GA — 250 60 — Comparativeexample 19 5 770 120 30 500 30 465 520 20 GA — 250 60 — Comparativeexample 20 E 6 750 120 6 520 300 465 — 20 GI 0.10 300 10 — Example 21 6750 120 6 520 300 465 — 20 GI — 300 10 0.20 Example 22 6 750 120 6 520300 465 — 20 GI 0.10 300 10 — Comparative example 23 6 750 120 6 380 300465 — 20 GI 0.10 300 10 — Comparative example 24 F 2 700 600 3 550 480465 480 30 GA — 150 7200 — Example 25 2 700 600 3 550 480 465 480 30 GA— 45 7200 Comparative example 26 2 700 600 3 550 480 465 480 30 GA — 4507200 — Comparative example 27 G 6 740 480 12 500 5 465 510 120 GA 0.40350 2 — Example 28 6 740 480 12 500 5 465 510 120 GA 0.40 350 2 —Example 29 H 10 780 60 15 500 180 465 550 40 GA — 270 600 — Example 30 I1 780 60 15 500 180 465 550 40 GA 0.10 200 36000 0.20 Example 31 J 5 78060 15 500 180 465 550 60 GA — 250 7200 0.20 Example 32 K 5 750 120 15500 180 465 550 60 GA — 300 120 — Comparative example 33 L 5 780 120 15500 180 465 550 60 GA — 300 120 — Comparative example 34 M 5 730 120 15500 180 465 500 60 GA — 300 120 — Comparative example 35 N 5 750 120 15500 180 465 550 60 GA — 300 120 — Comparative example 36 O 5 750 120 15500 180 465 510 60 GA — 300 120 — Comparative example 37 P 5 750 120 10500 60 465 550 20 GA — 250 7200 — Comparative example *Coated state: GI:galvanized steel sheet, GA: galvannealed steel sheet

TABLE 3 Steel Steel structure Mechanical properties sheet V(TM)*¹ V(F)*¹V(γ)*¹ Others*1 V(M)*¹ V(BF)*¹ d(TM)*² C(γ)*³ YS TS UEL Bend- No. (%)(%) (%) (%) (%) (%) (μm) (%) (MPa) (MPa) (%) ability Remarks  1 42 48 91 0 0 2 0.4 870 1182 10.1 good Example  2 23 70 5 2 0 0 2 0.4 695 11029.2 good Comparative example  3 43 49 7 1 0 0 9 0.4 865 1188 9.4 poorComparative example  4 29 55 4 12 0 0 2 0.3 793 1134 8.8 goodComparative example  5 28 58 3 11 0 0 3 0.3 789 1126 8.8 goodComparative example  6 52 43 4 1 0 1 4 0.3 907 1231 9.1 good Example  729 68 3 0 0 0 4 0.3 805 1140 8.9 good Comparative example  8 58 39 3 0 00 9 0.2 939 1268 7.9 poor Comparative example  9 19 72 9 0 0 0 1 0.3 684967 10.9 good Comparative example 10 62 21 2 15 0 15 6 0.2 995 1302 6.5good Comparative example 11 75 24 1 0 0 0 7 0.2 1016 1353 6.1 goodComparative example 12 52 39 6 3 0 0 4 0.4 1185 1366 9.4 good Example 1328 64 4 4 0 0 4 0.4 839 1175 9.9 good Comparative example 14 61 31 3 5 02 9 0.4 1117 1371 7.2 poor Comparative example 15 53 34 3 10 0 7 4 0.41123 1359 6.9 good Comparative example 16 48 48 3 1 0 1 3 0.5 951 12939.5 good Example 17 36 53 1 10 0 0 3 0.3 896 1192 6.6 good Comparativeexample 18 21 48 6 25 0 25 3 0.4 877 1116 9.8 good Comparative example19 50 44 1 5 0 5 4 0.4 965 1296 6.9 good Comparative example 20 38 52 82 0 2 2 0.6 955 1257 8.9 good Example 21 38 53 7 2 0 2 2 0.6 946 12548.8 good Example 22 43 50 6 1 0 1 10 0.6 933 1263 8.8 poor Comparativeexample 23 30 51 4 15 0 15 2 0.5 882 1177 8.3 good Comparative example24 32 55 13 0 0 0 1 0.1 855 1229 13.8 good Example 25 2 55 13 30 30 0 10.1 733 1288 13.5 poor Comparative example 26 32 55 1 12 0 3 1 0.1 9421126 5.8 good Comparative example 27 40 53 4 3 0 2 2 0.5 899 1230 7.7good Example 28 41 52 4 3 0 2 2 0.5 903 1234 7.5 good Example 29 58 35 34 0 2 3 0.4 962 1281 7.2 good Example 30 61 35 2 2 0 1 3 0.3 1043 13107.3 good Example 31 63 33 3 1 0 0 3 0.3 1022 1316 7.9 good Example 32 3855 2 5 0 3 2 0.4 860 1099 8.8 good Comparative example 33 41 44 9 6 0 04 0.5 856 1346 10.3 poor Comparative example 34 48 45 1 6 0 3 1 0.3 8791249 6.7 good Comparative example 35 37 48 1 14 0 8 2 0.4 880 1160 6.9good Comparative example 36 39 60 1 0 0 0 1 0.4 875 1178 8.2 poorComparative example 37 53 20 27 0 0 0 2 0.4 1024 1463 6.8 poorComparative example *¹V(TM), V(F), V(γ), V(M), V(BF), and others: Thearea fractions of a tempered martensite phase, a ferrite phase, aretained austenite phase, a martensite phase, and a bainitic ferritephase, respectively. Others: the area fraction of phases including V(M),V(BF), and a phase other than the foregoing phases. *²d(TM): The averagegrain size of the tempered martensite phase. *³C(γ): The C content ofthe retained austenite phase.

Any of the high-strength galvanized steel sheets according to theexamples of the present disclosure have a tensile strength (TS) of 1180MPa or more, a yield strength (YS) of 850 MPa or more, a uniformelongation of 7.0% or more, and good bendability. In the comparativeexamples, which are outside the scope of the present disclosure, adesired tensile strength (TS) is not obtained, a desired yield strength(YS) is not obtained, a desired uniform elongation is not obtained, or adesired bendability is not obtained.

INDUSTRIAL APPLICABILITY

According to the present disclosure, a high-strength galvanized steelsheet having a tensile strength (TS) of 1180 MPa or more, a yieldstrength (YS) of 850 MPa or more, a uniform elongation of 7.0% or more,and good bendability is provided. The use of the high-strengthgalvanized steel sheet of the present disclosure for automobile partscontributes to a reduction in the weight of automobiles to markedlycontribute to an increase in the performance of automobile bodies.

1. A high-strength galvanized steel sheet, the steel sheet having achemical composition comprising: C: 0.15% or more and 0.25% or less, bymass %; Si: 0.50% or more and 2.5% or less, by mass %; Mn: 2.3% or moreand 4.0% or less, by mass %; P: 0.100% or less, by mass %; S: 0.02% orless, by mass %; Al: 0.01% or more and 2.5% or less, by mass %; and Feand inevitable impurities, wherein the steel sheet has a microstructurecontaining, in terms of area fraction: a tempered martensite phase: 30%or more and 73% or less, a ferrite phase: 25% or more and 68% or less, aretained austenite phase: 2% or more and 20% or less, and other phases:10% or less (including 0%), the other phases containing a martensitephase: 3% or less (including 0%) and a bainitic ferrite phase: less than5% (including 0%), the tempered martensite phase having an average grainsize of 8 μm or less, and the retained austenite phase having a Ccontent of less than 0.7% by mass.
 2. The high-strength galvanized steelsheet according to claim 1, wherein the chemical composition furthercomprises at least one group selected from the groups consisting of A,B, and C: group A—at least one element selected from: Cr: 0.01% or moreand 2.0% or less, by mass %, Ni: 0.01% or more and 2.0% or less, by mass%, and Cu: 0.01% or more and 2.0% or less, by mass %, group B—B: 0.0002%or more and 0.0050% or less, by mass %, and group C—at least one elementselected from: Ca: 0.001% or more and 0.005% or less, by mass %, andREM: 0.001% or more and 0.005% or less, by mass %.
 3. (canceled) 4.(canceled)
 5. The high-strength galvanized steel sheet according toclaim 1, wherein the galvanized steel sheet includes a galvannealedsteel sheet.
 6. The high-strength galvanized steel sheet according toclaim 1, wherein the galvanized steel sheet has a tensile strength of1180 MPa or more.
 7. A method for producing a high-strength galvanizedsteel sheet, the method comprising: a hot-rolling step of heating a slabto a temperature of 1100° C. or higher, hot-rolling the slab at a finishrolling temperature of 800° C. or higher to produce a hot-rolled steelsheet, and coiling the hot-rolled steel sheet at a coiling temperatureof 550° C. or lower, the slab having a chemical composition comprising:C: 0.15% or more and 0.25% or less, by mass %, Si: 0.50% or more and2.5% or less, by mass %, Mn: 2.3% or more and 4.0% or less, by mass %,P: 0.100% or less, by mass %, S: 0.02% or less, by mass %, Al: 0.01% ormore and 2.5% or less, by mass %, and Fe and inevitable impurities; acold-rolling step of cold-rolling the hot-rolled steel sheet at acumulative rolling reduction of more than 20% to produce a cold-rolledsteel sheet; a pre-annealing step of heating the cold-rolled steel sheetto an annealing temperature of 800° C. or higher and 1000° C. or lower,holding the cold-rolled steel sheet at the annealing temperature for 10s or more, and cooling the cold-rolled steel sheet to a cooling stoptemperature of 550° C. or lower at an average cooling rate of 5° C./s ormore; a final-annealing step of heating the cold-rolled steel sheet toan annealing temperature of 680° C. or higher and 790° C. or lower at anaverage heating rate of 10° C./s or less, holding the cold-rolled steelsheet at the annealing temperature for 30 s or more and 1000 s or less,cooling the cold-rolled steel sheet to a cooling stop temperature of460° C. or higher and 550° C. or lower at an average cooling rate of1.0° C./s or more, and holding the cold-rolled steel sheet at thecooling stop temperature for 500 s or less; a galvanization step ofgalvanizing the cold-rolled steel sheet subjected to final annealing andcooling the galvanized cold-rolled steel sheet to room temperature; anda tempering step of tempering the galvanized cold-rolled steel sheet ata tempering temperature of 50° C. or higher and 400° C. or lower.
 8. Themethod for producing a high-strength galvanized steel sheet according toclaim 7, wherein the galvanization step further includes, after thegalvanization, galvannealing treatment in which the galvanizedcold-rolled steel sheet is held in a temperature range of 460° C. orhigher and 580° C. or lower for 1 s or more and 120 s or less and thencooled to room temperature.
 9. The method for producing a high-strengthgalvanized steel sheet according to claim 7, further comprising, beforeand/or after the tempering step, a temper-rolling step of temper rollingat an elongation rate of 0.05% or more and 1.00% or less.
 10. The methodfor producing a high-strength galvanized steel sheet according to claim8, further comprising, before and/or after the tempering step, atemper-rolling step of temper rolling at an elongation rate of 0.05% ormore and 1.00% or less.
 11. The method for producing a high-strengthgalvanized steel sheet according to claim 7, wherein the steel sheet hasa microstructure containing, in terms of area fraction: a temperedmartensite phase: 30% or more and 73% or less, a ferrite phase: 25% ormore and 68% or less, a retained austenite phase: 2% or more and 20% orless, and other phases: 10% or less (including 0%), the other phasescontaining a martensite phase: 3% or less (including 0%) and a bainiticferrite phase: less than 5% (including 0%), the tempered martensitephase having an average grain size of 8 μm or less, and the retainedaustenite phase having a C content of less than 0.7% by mass.
 12. Themethod for producing a high-strength galvanized steel sheet according toclaim 7, wherein the chemical composition further comprises at least onegroup selected from the groups consisting of A, B, and C: group A—atleast one element selected from: Cr: 0.01% or more and 2.0% or less, bymass %, Ni: 0.01% or more and 2.0% or less, by mass %, and Cu: 0.01% ormore and 2.0% or less, by mass %, group B—B: 0.0002% or more and 0.0050%or less, by mass %, and group C—at least one element selected from: Ca:0.001% or more and 0.005% or less, by mass %, and REM: 0.001% or moreand 0.005% or less, by mass %.
 13. The high-strength galvanized steelsheet according to claim 2, wherein the galvanized steel sheet includesa galvannealed steel sheet.
 14. The high-strength galvanized steel sheetaccording to claim 2, wherein the galvanized steel sheet has a tensilestrength of 1180 MPa or more.
 15. The high-strength galvanized steelsheet according to claim 5, wherein the galvanized steel sheet has atensile strength of 1180 MPa or more.
 16. The high-strength galvanizedsteel sheet according to claim 13, wherein the galvanized steel sheethas a tensile strength of 1180 MPa or more.
 17. The method for producinga high-strength galvanized steel sheet according to claim 12, whereinthe galvanization step further includes, after the galvanization,galvannealing treatment in which the galvanized cold-rolled steel sheetis held in a temperature range of 460° C. or higher and 580° C. or lowerfor 1 s or more and 120 s or less and then cooled to room temperature.18. The method for producing a high-strength galvanized steel sheetaccording to claim 12, further comprising, before and/or after thetempering step, a temper-rolling step of temper rolling at an elongationrate of 0.05% or more and 1.00% or less.
 19. The method for producing ahigh-strength galvanized steel sheet according to claim 17, furthercomprising, before and/or after the tempering step, a temper-rollingstep of temper rolling at an elongation rate of 0.05% or more and 1.00%or less.